In the minerals industry, occurrences of ore bodies or parts of ore bodies, which are either too small in total reserves or too low in grade to justify milling or other conventional processing are common. Also, many such ore bodies occur long distances from existing milling or processing facilities. Commonly these ore deposits have been either left untreated or have been treated by processes requiring a relatively low capital investment. Such processes include heap, vat, or in situ leaching. Unfortunately, these processes usually sacrifice percentage and rate of recovery in return for the benefit of lower capital investment and operating costs. And, the overall economics are often marginal at best. In addition, recent environmental and regulatory restrictions, particularly in the uranium industry, such as tailings disposal and site reclamation, have further diminished the opportunity for profitably developing such small or low grade ore bodies.
It is known in the prior art to cure metal containing ores and then subject them to a leaching process to remove metal values. Nevertheless, the prior art has no contemplation or awareness of Applicants' particular solution to the problem, which accomplishes an extraction of metal values from low quality ores in an expedient and economic manner. Moreover, Applicants' technique can be accomplished with portable equipment.
The following represents an analysis of the most significant references known to Applicants. These are:
U.S. Pat. No. 3,269,832 to Abell et al discloses a process for the extraction of metal values, which comprises comminuting a metal-containing ore and moistening the resulting crushed ore with a small amount of leachant, such as sulfuric acid to total moisture content of between 6 and about 18 percent. Thereafter, the moistened ore is allowed to stand in a reactor tank for at least one hour and up to 24 hours. The thus-cured ore in a relatively thick layer is thereafter countercurrently washed with no agitation and metal values are extracted from the wash water.
Abell's process stresses agglomerization of fine ore particles as a critical feature to limit the detrimental effect of slimes on liquid flow rate. This process will not operate without agglomeration. The ore is never pulped but rather maintained in the agglomerated state, followed by quiescent submergence in wash solution and then draining off the wash solution by gravity flow. The mechanism by which this is accomplished is complex, bulky, and awkward. Gravity flow wash is critical, as opposed to forced flow, e.g., suction filtration, which is expressly disclaimed. Forced flow in Abell's process causes undesirable reduction of wash liquid flow.
U.S. Pat. No. 4,017,309 to Johnson discloses a layer-leaching method for the recovery of metal values from metal-containing ores. Although Johnson characterizes his method as concerning a "thin" layer, actually it is a relatively thick layer.
This method comprises crushing the ore, and adding a leachant such as sulfuric acid, adjusting the moisture content to within the range of about 8 to 18 percent by weight, and thereafter allowing the ore to cure for about two days. Subsequently, the cured ore is spread upon an impermeable leach pad, preferably in a layer about one-half to one and one-half meters thick and a weak leach solution is sprayed over the surface of the layer of ore. Metal values are slowly leached from the ore as the solution passes through. The dissolved metal values are subsequently recovered from the pregnant liquor. The thickness of the thin layer of ore is said to be critical and is much thicker than that of Applicants'. Moreover, Johnson requires induration, e.g., hardening and strengthening of his ore particles as a critical requirement for his process during the cure stage. (Induration is accomplished through agglomeration.)
In Applicants' process, agglomeration is undesirable because it tends to prevent achievement of very fast recovery of metal values.